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Thread: Is resistance transferable ?

  1. #11
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    Upsize EGC

    I agree that upsizing to a larger conductor for the 1000ft run to lower the voltage drop is not a problem and solves the issue. However, NEC requires that the EGC be upsized if the conductors are upsized to reduce voltage drop. So, if you run #12 AWG for hot, neutral and ground then switch to 1/0 for the 1000ft, then the hot, neutral and ground all need to be upsized to 1/0.

    For a 1000ft run, you are better off stepping up to 600V then back down again. But, that is not the question here.
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  2. #12
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    You also are allowed to do that splice in the panel itself, you don't need to add a gutter.
    The world is round, you will get there no matter what path you take.

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    Quote Originally Posted by rlundsrud View Post
    You also are allowed to do that splice in the panel itself, you don't need to add a gutter.
    Unless there isn't sufficient space to do so. Small 100 amp panel and 350 kcmil conductors because of voltage drop - may not be even be deep enough panel for the required raceway to enter the cabinet let alone any room to do anything with those conductors if you do get them in there.
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    Quote Originally Posted by beanland View Post
    I agree that upsizing to a larger conductor for the 1000ft run to lower the voltage drop is not a problem and solves the issue. However, NEC requires that the EGC be upsized if the conductors are upsized to reduce voltage drop. So, if you run #12 AWG for hot, neutral and ground then switch to 1/0 for the 1000ft, then the hot, neutral and ground all need to be upsized to 1/0.
    Not necessarily. The ground needs to be upsized proportionately from its minimum size as dictated by code, not necessarily from what it is in the small wire run.

  5. #15
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    Quote Originally Posted by drcampbell View Post
    "Transferable" is the wrong word and might be causing confusion..
    Additive or cumulative might be better words for what amounts to a series circuit?

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    Quote Originally Posted by ggunn View Post
    Not necessarily. The ground needs to be upsized proportionately from its minimum size as dictated by code, not necessarily from what it is in the small wire run.
    Thing is normally a 20 amp circuit uses 12 AWG circuit conductors and requires 12 AWG EGC. Since they are both already same size, an increase because of voltage drop means both are still same size afterwards if they were increased the same proportion. Only when the EGC was smaller to begin with will result in still having smaller EGC, but still proportionally larger.
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  7. #17
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    190518-1626 EDT

    Look at the problem from a circuit perspective.

    Assume a breaker or fuse opens within 1/60 second at 10 times its rating. Just to make things simple. Also assume our criteria is circuit interruption within 1/60 second with a dead short circuit at the circuit end.

    Consider a #12 copper circuit 1000 ft long with #12 copper for the EGC protected at 20 A. Loop resistance is about 3 ohms. Short circuit current at 120 V input is 40 A. Does not meet our criteria of 200 A for instantaneous trip. So independent of voltage drop we can not use #12.

    Next assume hot and neutral are increased to #6. A six number size change or about 1/4 the resistance, 1/2*1/2. So 1.5/4 makes the hot about 3/8 ohm. Loop resistance for a dead short at end of hot to EGC is 0.375 + 1.5 = 1.875, or a 120 V current of 64 A. Does not meet our criteria.

    Next make the EGC equal to the hot, and loop resistance becomes 0.375 + 0.375 = 0.75 ohms. Now 120 V short circuit current is 120 / (3/4) = 160 A. This still does not meet our criteria, but it is a lot closer.

    To meet our assumed criteria relative to short circuit current our wire size for all of hot, neutral (common or grounded is a better term), and EGC needs to be larger. Note, this is independent of needs for voltage drop.

    I have not rechecked my calculations, but the theory illustrates the point.

    .

  8. #18
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    Quote Originally Posted by kwired View Post
    Thing is normally a 20 amp circuit uses 12 AWG circuit conductors and requires 12 AWG EGC. Since they are both already same size, an increase because of voltage drop means both are still same size afterwards if they were increased the same proportion. Only when the EGC was smaller to begin with will result in still having smaller EGC, but still proportionally larger.
    Of course. I was speaking to the general case, i.e., if the CCC's and EGC are all the same size, increasing the size of the CCC's for Vd does not necessarily mean you have to increase the size of the EGC. The EGC may already be oversized.
    Last edited by ggunn; 05-19-19 at 05:24 PM.

  9. #19
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    Quote Originally Posted by gar View Post
    190518-1626 EDT

    Look at the problem from a circuit perspective.

    Assume a breaker or fuse opens within 1/60 second at 10 times its rating. Just to make things simple. Also assume our criteria is circuit interruption within 1/60 second with a dead short circuit at the circuit end.

    Consider a #12 copper circuit 1000 ft long with #12 copper for the EGC protected at 20 A. Loop resistance is about 3 ohms. Short circuit current at 120 V input is 40 A. Does not meet our criteria of 200 A for instantaneous trip. So independent of voltage drop we can not use #12.

    Next assume hot and neutral are increased to #6. A six number size change or about 1/4 the resistance, 1/2*1/2. So 1.5/4 makes the hot about 3/8 ohm. Loop resistance for a dead short at end of hot to EGC is 0.375 + 1.5 = 1.875, or a 120 V current of 64 A. Does not meet our criteria.

    Next make the EGC equal to the hot, and loop resistance becomes 0.375 + 0.375 = 0.75 ohms. Now 120 V short circuit current is 120 / (3/4) = 160 A. This still does not meet our criteria, but it is a lot closer.

    To meet our assumed criteria relative to short circuit current our wire size for all of hot, neutral (common or grounded is a better term), and EGC needs to be larger. Note, this is independent of needs for voltage drop.

    I have not rechecked my calculations, but the theory illustrates the point.

    .
    As the current for a short in #12 copper 1000 ft long circuit is only 40A, it may be considered a overload current instead of short circuit current for the thermal-magnetic CB and the tripping time may be found from the overload characteristic curve of the CB accordingly.

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    190520-0739 EDT

    Sahib:

    Your statement is correct, but did you bother to read my assumptions which were there to define or frame the background for the scope of my discussion?

    .

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